I feel your pain
◑‿◑ (hey, others might find their way here).
One thing to check is the perceived type for images. For example, to ensure that the type for JPEGs is correct, check the registry for the following (or modify if necessary):
However this may not work (it didn’t for me) and the setting may already be correct.
Another, easier, faster, simpler, and safer way to fix this issue (which worked for me) is to simply re-register Windows built-in image-handling shell-extension (should work in Windows 7 as well):
You’ll need to restart Explorer for it to take effect (reboot, log out and in, or kill Explorer and run it).
As to why it wasn’t working, I don’t know. Obviously some setting (likely in the registry) had been changed (e.g., a program associated various image types). It may also have been changed by disabling a file-handler or some such. If I get some time to do some experiments, I’ll try to determine the exact setting, but fixing it is as simple as running the above command (and restarting Explorer).